Same thing happened in summer of 2021 to a friend who owns an Oceanis 321. What broke in 2 was the aluminum part. We undid it and found part number # 031514 underneat. Check on Beneteau's site, yours could be the same number. Part is called Stem Head Fitting Cast Aluminum by Beneteau. They had it in stock but it was quite expensive. At that time it was US$ 1122.34. At that time we also had found Klacko, a company in St.Catharines, Ontario, Canada that made an exact replica of Beneteau 031514 but in stainless. We never received a quote from them.
In the end, since the boat was going to be on the hard with mast down for the entire winter season, my friend took the fitting (broken in 2 parts) and the pictures to a very good aluminum welder who guaranteed his work. All for $100 Canadian! In the Spring we reinstalled the welded Stem Head Fiting and re-stepped the mast. Boat has been extensively used now during summers of 22 and 23, mast has been stepped and unstepped every Spring and Fall, and the Stem Head fitting looks as good as new. The fitting on the 321 is not part of the chainplate for the headstay. If yours is the same design or the same part, my advice would be to look for a specialist in aluminum welding, and take the fitting there. On the 321, it was not hard to remove the fitting as it is held with only 3 bolts and you can reach the nuts from inside the anchor well. If I was you and I didn't want to take the mast down, the way the fitting is made I would loosen the backstay and the shrouds a little bit, secure the mast to the bow cleats with the head sail and spinnaker halyards, remove the head stay, remove the fitting and have it welded, then re-install everything and retune the mast. Good luck